Diagnostic Procedure

Diagnosis of the computerized engine control system should be performed in the following order:

1) Ensure all engine systems not related to the computer are operating properly. DO NOT proceed with testing unless all other problems have been repaired. Appropriate diagnostic circuit check MUST be performed before using trouble code charts. See BASIC TESTING article.

2) If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes cause SERVICE ENGINE SOON light to illuminate continuously with engine running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION in this article. For diagnosing hard codes, proceed to appropriate trouble code chart in this article. For diagnosing intermittent codes, proceed to INTERMITTENTS in TESTS W/O CODES article. Exceptions are Code 13, 15, 24, 44 and 45 charts, which can help diagnose intermittent codes.

3) If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in TESTS W/O CODES article. From there you will be sent to the appropriate area in SYSTEMS & COMPONENT TESTING article.

4) After repairs are made, clear trouble codes and perform SYSTEM PERFORMANCE CHECK or FIELD SERVICE MODE check in BASIC TESTING article.